Inhomogeneity in body fat distribution may result in inaccuracy in the measurement of vertebral bone mass.

When bone mineral content (BMC) is measured by dual X-ray absorptiometry (DXA), the X-ray beam is attenuated by bone and soft tissue. Since the component of the attenuation caused by the soft tissue overlying bone cannot be measured, the attenuation caused by soft tissue adjacent to bone is measured and is used in the calculation of BMC. the assumption underlying this approach is that the amount and composition of this adjacent soft tissue is the same as overlying bone. The aim of this study was to examine the validity of this assumption by determining whether fat distribution over and adjacent to bone differ and whether this introduces accuracy errors in the measurement of BMC by postero-anterior (PA) and lateral scanning. BMC (posterior processes plus vertebral body, g) of the third lumbar vertebra was 17.3 +/- 0.7 by PA and 17. +/- 0.7 by lateral scanning in 27 premenopausal women (p = NS), but 2.7 g or 20% higher by PA than scanning in 27 postmenopausal women (14.4 +/- 0.7, 11.7 +/- 0.5, p<0.01). Thus, the respective diminutions across age by PA scanning was about half that by lateral scanning (16.8 +/- 3.9%, 31.2 +/- 3.0%, p<0.01). Percent fat in the soft tissue baseline (anterior to bone, ST-ant) used to derive BMC by lateral scanning by 2.6 +/- 0.7% in premenopausal women and 7.5 +/- 1.0% in postmenopausal women (both p<0.01). After adjusting for these differences in percent fat, BMC by PA and lateral scanning no longer differed.